Resonant pumped short cavity fiber laser

ABSTRACT

A fiber laser or fiber amplifier uses resonant pumping of the gain medium by providing a pump resonator that establishes a resonator cavity at the pump wavelength which includes the pumped gain medium. The pump resonator may be of a distributed feedback (DFB) or a distributed Bragg reflector (DBR) type construction, and may be combined with signal reflection apparatus of either DFB or DBR type construction that provides oscillation of the desired laser output wavelength. If used without a signal reflection apparatus, the invention may be operated as a resonant pumped fiber amplifier. Resonant pumped lasers may be arranged in series to provide a laser apparatus with a selectable output wavelength. The different laser stages each provide resonance for a different pump wavelength, and each provide resonance for a different signal wavelength. Thus, a particular pump wavelength input to the series arrangement only resonates in one of the laser stages, and therefore only pumps the gain medium of that stage, allowing it to output its unique signal wavelength. The output signals from each laser stage may be coupled out of the series arrangement with wavelength selective optical couplers, each associated with a different laser stage. A resonant pumped laser may use a wavelength stabilized pump source to maximize pumping efficiency, the stabilization being provided by a feedback signal, preferably an electrical feedback signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 08/970,745,filed Nov. 14,1997, now U.S. Pat. No. 6,044,070.

FIELD OF THE INVENTION

This invention relates to the field of optical signal processing and,more particularly, to the generation of a low-noise, single frequencylaser output.

BACKGROUND OF THE INVENTION

In optical communications, it is desirable to use a communicationwavelength in the wavelength range of approximately 1.5 micrometers(μm). This is because 1.5 μm is the lowest loss wavelength ofconventional single-mode glass fibers. As the number of desired channelsand demand for high video quality increases, the corresponding demandson an optical transmission source also increase. Among these is thedemand for a low-noise, single-frequency 1.5 μm laser source. Fiberlaser sources have been created which use an erbium (Er) doped opticalfiber as a gain medium. However, such fiber lasers typically suffer froma tradeoff between gain and power, and mode discrimination.

It is commonly understood that, within a fiber laser, longitudinal modespacing is inversely proportional to the length of the laser cavity.That is, for a longer resonator cavity, the frequency spacing betweenadjacent resonance frequencies is smaller. As a result, morelongitudinal modes are within the (necessarily finite) width of the gainspectrum, and mode discrimination is poor. Even if the effective gainspectrum of the fiber laser is narrowed through the use of narrow bandreflectors, such as Bragg gratings written directly into the core of thefiber, many modes may exist with that effective gain bandwidth. For thedesigner of a single-frequency laser, this causes several difficulties.Firstly, the wide spectrum emitted from the laser interferes with otherlaser sources transmitted at different wavelengths in wavelengthdivision multiplexed systems. Secondly, dispersion in the wide spectrumresults in pulse spreading. Finally, mode competition betweenlongitudinal modes causes amplitude instability.

By making the laser cavity shorter, the mode spacing is increased and,if the cavity is short enough, the gain spectrum may be limited to asingle resonance frequency. However, the length of the gain medium (i.e.the doped fiber surrounding the cavity) also affects the optical gain ofthe laser. With a relatively short length of fiber, the gain of thelaser is low since there is less distance across which the pump energymay be absorbed within the fiber.

It would be desirable to have an optical source that has an output powersignificantly higher than previously available, and which has the modediscrimination benefits of a short fiber gain medium.

SUMMARY OF THE INVENTION

The present invention provides a single-frequency laser source in whicha high output power is achieved by developing a resonant condition forthe pump energy wavelengths within the laser cavity. In general, thelaser source has an optical fiber gain medium and a signal reflectionapparatus that reflects light at the signal wavelength within the gainmedium, so as to establish a resonance condition at the signalwavelength. The signal reflection apparatus is substantially entirelyreflective when it comes to optical energy in the cavity travelingtoward an input side of the cavity, and partially reflective to opticalenergy in the cavity traveling toward an output side of the cavity. Thispartial reflectivity provides the desired output coupling for the laser.The gain medium is pumped by a pump source that couples light at anappropriate pumping wavelength into the laser cavity. Once coupled intothe laser cavity, the pump energy is retained within the cavity using apump resonator that reflects light back and forth within a resonantcavity that includes the gain medium. This maximizes the pumpingefficiency, particularly for the short fiber laser, by retaining thepump energy within the gain medium.

The signal reflection apparatus may take different forms, as may thepump resonator. For example, the signal reflection apparatus may have adistributed feedback (DFB) type configuration in which a periodicgrating is built into a pumped part of the gain medium. As known in theart, a DFB grating has a shift in its structure that results in a{fraction (1/4+L )}-wavelength phase shift in light at the chosenwavelength (e.g., the signal wavelength), thus giving the desiredresonance condition. Alternatively, the signal reflection apparatus mayalso have a distributed Bragg reflector (DBR) type configuration, inwhich an input periodic grating is located relatively close to an inputside of the gain medium and an output periodic grating is locatedrelatively close to an output side of the gain medium. The input gratingis highly reflective to keep the optical energy at the signal wavelengthwithin the cavity, while the output grating is partially reflective toallow output coupling of the laser energy.

The configuration described above for the signal reflection apparatusmay be combined with different configurations for the pump resonator.The pump resonator may be a DFB type structure that is reflective at thepump wavelength and that is built into a part of the gain mediumoccupied by a DFB type signal reflection apparatus grating. The pumpresonator may also be a DBR type structure having two highly reflectiveperiodic gratings, one on either side of the signal reflectionapparatus, that reflect optical energy at the pump wavelength back andforth within the gain medium. The DBR type pump resonator may be usedwith either a DFB type or a DBR type signal reflection apparatus. Ineither circumstance, the gratings of the pump resonator surround thegrating or gratings of the signal reflection apparatus, and provide“resonant pumping” of the gain medium across the area occupied by thesignal reflection apparatus. Also, when a DBR type signal reflectionapparatus is used with a DBR type pump resonator, a Q-switch may also beused to generate a high-intensity, short-duration pulse.

In one variation of the invention, a DBR-type arrangement of pumpgratings may be used to construct a short fiber amplifier similar to theshort fiber lasers discussed above. However, rather than using a signalreflection apparatus which develops the desired signal wavelength suchthat it originates in the gain medium, only the pump resonator is used.The signal to be amplified is then introduced into the pumped gainmedium, where it is amplified before being output. The resonant pumpingof the gain medium maintains a desired level of population inversion,and allows the signal passing through the gain medium to be amplified bystimulated emission. Also, with the use of only a single, broadbandsignal reflector, an amplified spontaneous emission (ASE) source may beformed that provides a broadband optical output.

In one embodiment of the invention, a plurality of resonant-pumpedlasers are arranged in series and allow an optical signal to begenerated at any of a plurality of different signal wavelengths. Each ofthe laser cavities may have a different combination of DFB and DBR typesignal reflection apparatus and pump resonator. For each laser, a DFBtype signal reflection apparatus may be combined with either a DFB or aDBR type pump resonator, or a DBR type signal reflection apparatus maybe combined with a DBR type pump resonator. Each of the lasers in serieshas a signal reflection apparatus constructed to oscillate at a signalwavelength different than the signal wavelengths of the other lasers.Furthermore, each of the lasers has a pump resonator constructed tooscillate at a pump wavelength different than the pump wavelengths ofthe other lasers. Thus, when a first pump wavelength is directed intothe series arrangement of lasers, it resonates only in a first lasercavity that has its pump resonator constructed to oscillate at the firstpump wavelength. This results in the gain medium of the first lasercavity being resonant pumped, while the other laser cavities achieve nosuch pumping. The pumping of the first laser cavity causes thegeneration of signal energy within the first cavity, which resonates ata first desired signal wavelength, being reflected within the cavity bythe signal reflection apparatus and eventually coupled out of the firstlaser cavity. When a different pump wavelength is introduced into theseries laser arrangement, it oscillates in the pump resonator of adifferent one of the laser cavities, causing that laser cavity to beresonant pumped, and producing a laser output at a second desired signalwavelength. Thus, the wavelength of the laser output is selectable byselecting the wavelength of the input pump energy.

In one variation of the series laser embodiment described above, awavelength selective element is used at the output of any of the lasers.The wavelength selective element at the output of a given laser coupleslight at the signal wavelength of that laser (i.e. the wavelength atwhich the signal reflection apparatus of that laser is designed toresonate) to a first output port, while directing other wavelengthsthrough to any subsequent lasers of the series arrangement. This avoidsany losses associated with the output from a given laser passing throughsubsequent optical components.

In another embodiment of the invention, the pump wavelength isstabilized to prevent reduced efficiency in a resonant pumped laser dueto wavelength drift of the pump source. A first variation of thisembodiment uses pump light that has a time-varying wavelength due tomodulation of a electrical input signal to the optical signal generator,creating two small frequency modulation (FM) side band frequencies.Light reflected from the laser cavity is then used to generate afeedback. The reflected light has an intensity which is minimum (and thetransmitted light a maximum) when the light directed to the cavity has awavelength that satisfies the peak resonant condition of the cavity.When this peak resonance condition is satisfied, a relative time-varyingintensity phase between the reflected optical energy and the pump sourceoptical energy is also a minimum. However, when the wavelength of thepump source drifts away from the wavelength for which the pump resonatorof the laser achieves peak resonance, a phase shift develops between thetwo FM sidebands resulting in an FM-to-amplitude modulation (AM)conversion. The relative strength of this AM signal is a measure of howfar the pump laser is from its resonance condition. Therefore, thissignal may be used to control the wavelength of the pump laser.

The reflected pump energy is coupled to a photodetector that converts itto an electrical feedback signal having a magnitude proportional to theintensity of the reflected optical signal. To control the wavelength ofthe pump laser, this electrical signal may then, for example, be inputto an electrical mixer that combines it with an AC component of theelectrical input signal that drives the optical signal generator. Themixing of these two AC signals results in DC offset signal that iscombined with the electrical input signal to increase or reduce thedriving current to the optical signal generator, and thereby adjust itswavelength to maintain resonance. If the pump signal has drifted to alonger wavelength, the phase shift between the reflected optical signaland the pump source optical signal causes the DC offset to increase thedriving current of the optical signal generator and therefore shortenthe pump signal wavelength. Conversely, if the pump signal has driftedto a shorter wavelength, the phase shift between the reflected opticalsignal and the pump source signal causes the DC offset to decrease thedriving current of the optical signal generator, thereby lengthening thepump signal wavelength.

Preferably, the current driven component of the optical signal generatoris a semiconductor diode which is supplied with a DC current componentfrom a DC electrical power source and an AC current from an AC signalsource. Optionally, an optical filter may be used to block wavelengthsin the reflected optical energy which are outside the desired wavelengthrange. Also, the optical signal from the optical signal generator may bepassed through an optical isolator prior to its reaching the lasercavity, to avoid reflected signals from returning directly to the signalgenerator.

In another wavelength-stabilized variation of the invention, the pumpwavelength is stabilized using optical feedback from the laser cavity.The optical pump signal is transmitted to the laser cavity via anoptical circulator which receives the optical signal from a signalgenerator along a first optical path. While the majority of the opticalpump energy is contained within the laser cavity by the pump resonator,a small amount of this optical energy is also transmitted past the lasercavity. The amount of optical energy transmitted past the cavity ismaximum when the wavelength of the light from the pump signal generatorsatisfies the peak resonance condition of the pump apparatus (i.e.exactly matches the wavelength for which the grating or gratings of thepump resonator were designed). The transmitted optical energy is coupledback to the optical circulator, which directs it to the first opticalpath in a propagation direction opposite to the propagation direction ofthe signal from the signal generator. Thus, the transmitted opticalsignal is returned to the signal generator as an optical feedbacksignal. The signal generator locks onto the frequency of the feedbacksignal, and resonates at the frequency of the feedback signal. Thus,satisfying the peak resonant condition of the laser cavity provides thestrongest feedback signal, and the strongest lock on the wavelength ofthe pump signal output by the optical signal generator.

The optical signal generator of the optical feedback embodiment ispreferably a fiber laser that includes a doped optical fiber pumped witha semiconductor laser diode. Coupling of the transmitted optical energyback to the optical circulator is preferably accomplished using awavelength selective optical coupler, such as a wavelength divisionmultiplexer (WDM). The WDM couples only wavelengths in the range of thepump wavelength to be directed back to the optical circulator, whileother wavelengths (such as the signal wavelength) are directed to asignal output port. An optical filter may also be used between thedirectional coupler and the optical circulator which blocks opticalenergy outside of the wavelength range of the pump wavelength.Optionally, the magnitude of the transmitted optical energy may also beattenuated with an optical attenuator in an optical path between thedirectional coupler and circulator.

In another variation of the invention, multiple laser cavities can bepumped using a single pumping source. The laser cavities may beconstructed to each have a different output signal wavelength and apredetermined distribution of output powers. In the preferredembodiment, the output powers of all the different lasers are equal. Thelasers are all fed from a single pump source using a multiple branchoptical coupler. The optical coupler may be a fused fiber opticalcoupler, and receives the optical pump energy from the pump source,distributing it to each of the laser cavities. Each of the laserresonators has a gain medium which may be pumped by the optical energyat the wavelength of the energy from the pump source. However, eachresonates at a different signal wavelength. Thus, each of the lasersgets an equal distribution of pump power, but outputs a differentwavelength. In one embodiment the lasers are resonant pumped using pumpgratings on each which retain the pump energy within the gain medium.Furthermore, the stability of the pumping source may be stabilized usingeither electronic or optical feedback.

In a construction similar to that described above, a multiple branchdirectional coupler may also be used to couple a plurality of differentlaser outputs, each having a different wavelength, into a signal opticalwaveguide. The laser cavities are each located in a different gainmedium, each of which is coupled into a single waveguide by thedirectional coupler. The coupler may be a fused fiber coupler, and pumpenergy from a single pump source is coupled into the laser cavities fromthe single waveguide through the directional coupler. The pump sourcemay be a semiconductor laser diode. A wavelength selective coupler(preferably a WDM) is used to direct optical energy from the pump sourceinto the waveguide, while preventing energy outside the wavelength rangeof the pump wavelength from reaching the pump source. In one embodimentthe lasers are resonant pumped using pump gratings on each which retainthe pump energy within the gain medium. Furthermore, the stability ofthe pumping source may be stabilized using either electronic or opticalfeedback.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of a resonant pumped fiber laser accordingto the present invention that uses a signal energy reflector and a pumpenergy reflector each of which are of a distributed feedback (DFB) typeconstruction.

FIG. 1B is a schematic view of a resonant pumped fiber laser accordingto the present invention that uses a signal energy reflector having aDFB type construction and a pump energy reflector having a distributedBragg reflector (DBR) type construction.

FIG. 1C is a schematic view of a resonant pumped fiber laser accordingto the present invention that uses a signal energy reflector and a pumpenergy reflector each of which has a DBR construction.

FIG. 1D is a schematic view of a resonant pumped fiber laser similar tothat show in FIG. 1C, but is Q-switched.

FIG. 1E is a schematic view of a resonant pumped fiber amplifieraccording to the present invention.

FIG. 1F is a schematic view of a resonant pumped amplified spontaneousemission source according to the present invention.

FIG. 2A is a schematic view of a resonant pumped laser apparatusaccording to the present invention with a selectable output wavelength,the apparatus having two resonant cavities in series, each of which hasa construction as shown in FIG. 1A.

FIG. 2B is a schematic view of a resonant pumped laser apparatusaccording to the present invention with a selectable output wavelength,the apparatus having two resonant cavities in series, each of which hasa construction as shown in FIG. 1B.

FIG. 2C is a schematic view of a resonant pumped laser apparatusaccording to the present invention with a selectable output wavelength,the apparatus having two resonant cavities in series, each of which hasa construction as shown in FIG. 1C.

FIG. 3A is a schematic view of a resonant pumped laser similar to thatshown in FIG. 2B, but which also includes a wavelength selective outputcoupler for resonant cavities of the laser to allow low-loss outputcoupling of the laser outputs.

FIG. 3B is a schematic view of a resonant pumped laser similar to thatshown in FIG. 2C, but which also includes a wavelength selective outputcoupler for resonant cavities of the laser to allow low-loss outputcoupling of the laser outputs.

FIG. 4A is a schematic view of a resonant pumped laser that useselectrical feedback to stabilize the wavelength output of the opticalpumping source.

FIG. 4B is a schematic view of a resonant pumped laser that uses opticalfeedback to stabilize the wavelength output of the optical pumpingsource.

FIG. 5A is a schematic view of a multiple output laser that has multiplelaser resonators pumped by a single optical pumping source.

FIG. 5B is a schematic view of a laser source which has multiple laserresonators pumped by a single optical pumping source, and which combinesthe discrete wavelength outputs of the different resonators on a singleoptical waveguide.

FIG. 6A is a schematic view of a multiple output laser that has multiplelaser resonators that are resonant pumped and use a single opticalpumping source that is wavelength stabilized using electronic feedback.

FIG. 6B is a schematic view of a laser source having an output which isa combination of a number of discrete wavelengths, the laser sourcehaving multiple laser resonators that are resonant pumped and use asingle optical pumping source that is wavelength stabilized usingelectronic feedback.

FIG. 7A is a schematic view of a multiple output laser that has multiplelaser resonators that are resonant pumped and use a single opticalpumping source that is wavelength stabilized using optical feedback.

FIG. 7B is a schematic view of a laser source having an output which isa combination of a number of discrete wavelengths, the laser sourcehaving multiple laser resonators that are resonant pumped and use asingle optical pumping source that is wavelength stabilized usingoptical feedback.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Shown in FIG. 1A is a schematic depiction of a short fiber laser 11consisting of doped optical fiber 10 in which are etched two gratings.The gratings consist of a first grating (the “signal” grating) 12 and asecond grating (the “pump” grating) 14. Each of the gratings is awavelength-selective reflector etched into the cladding layer of thefiber 10, and each therefore reflects light at a predeterminedwavelength. The arrangement of FIG. 1A is similar to a “distributedfeedback” (DFB) laser structure, which is known in the art. In a DFBlaser, a periodic grating is built right into a pumped part of the gainregion. The grating is a periodic variation of refractive index alongthe gain medium that causes reflection at a desired signal wavelength.As is know in the art, the periodic variation includes a shift atroughly its center, which results in a phase shift of one quarter of thepredetermined wavelength. Thus, the structure of FIG. 1A uses a“λ/4-shifted” grating structure, as is common in DFB lasers. This allowspositive feedback of the optical energy at the signal wavelength, suchthat the desired resonance condition is achieved. Unlike the traditionalDFB laser, however, the FIG. 1A embodiment uses not only signal grating12, which is reflective at the signal wavelength, but also pump grating14, which is reflective at the wavelength of the optical energy fromoptical pumping source 15, which is used to pump the optical fiber.

In the embodiment of FIG. 1A, both of the gratings 12, 14 extend alongthe gain portion of the fiber 10, occupying the same longitudinalsection of fiber. The pump grating 14 has a DFB-type structure like thatof the signal grating 12, but it is constructed to be reflective at thepump energy wavelength. The result of this positioning of the twogratings is that, within the gain portion of the fiber, the gratingsprovide a resonant condition for both the signal energy and the pumpenergy. The signal wavelength λ_(s), is reflected within the gain regionby the signal grating 12, while the pump wavelength λ_(p) is reflectedwithin the gain region by the pump grating 14. The reflection of λ_(s)provides the desired resonance condition, as in a conventional fiberlaser. The reflection of λ_(p), meanwhile, contains the pump energywithin the gain portion of the optical fiber 10, where it may beabsorbed by the doped fiber material, allowing the stimulated emissionof optical energy at the signal wavelength λ_(s). This “resonantpumping” of the short fiber laser allows it to develop higher signalpowers than conventional short fiber lasers in which the pump energysimply passes once through the gain medium.

The embodiment of FIG. 1B is also a resonant pumped short fiber laser11, but rather than using a pump grating that extends the length of thegain medium, two pump gratings 14 are used, one on either side of thelaser cavity. The cavity is defined by the location of signal grating12, which is essentially the same as the signal grating 12 of FIG. 1A.The gratings 14 cause pump energy to be reflected back and forth throughthe doped fiber in the region containing the signal grating 12, suchthat the pump energy is absorbed, and spontaneous emission of opticalenergy at the signal wavelength λ_(s) results. This arrangement of thepump gratings is similar to that of a distributed Bragg reflector (DBR)laser in that the pump gratings are positioned to either side of theactive region of the laser.

Shown in FIG. 1C is another embodiment of the short fiber laser 11. Thisembodiment is again similar to a DBR laser in that the signal gratings12 are both positioned outside of the active region of the laser. As inthe FIG. 1B embodiment, the pump gratings 14 are also outside of theactive region, and the resulting structure provides resonance within thelaser cavity at both the signal wavelength λ_(s), and at the pumpwavelength λ_(p). The optical signal energy and the pumping energy isreflected back and forth within the cavity, each between its respectiveset of wavelength-selective gratings. The erbium-doped fiber 10 absorbsthe optical energy at the pump wavelength λ_(p), and spontaneousemission of optical energy at the signal wavelength λ_(s) results ingeneration and amplification of the optical signal, which resonateswithin the cavity established by the signal gratings. Thus, wherein theFIG. 1B embodiment uses a DFB type signal reflection apparatus with aDBR type pump resonator, the FIG. 1C embodiment uses a DBR type signalreflection apparatus with a DBR type pump resonator.

FIG. 1D is yet another embodiment of a resonant-pumped short fiber laseraccording to the present invention. The FIG. 1D embodiment is verysimilar to that of FIG. 1C, using a DBR type signal reflection apparatus12 with a DBR type pump resonator 14. Unlike FIG. 1C, however, theembodiment of FIG. 1D also includes a Q-switch 13. The concept ofQ-switching is well-known in the art, and involves inhibitingoscillation within the resonator cavity until a significant populationinversion has developed, at which time the obstacle to feedback isremoved and the laser quickly develops a high degree of oscillation anda high power, short duration (“giant”) pulse is emitted. The Q-switch 13of FIG. 1D may be any of a number of known types of Q-switch, and ispositioned between the signal gratings 12. In this embodiment, if a 1550nm signal wavelength is used, and the fiber is erbium-doped, it ispreferred that the pump wavelength be 1480 nm, that is, the gratings 14in FIG. 1D are constructed to have a peak resonant condition atapproximately 1480 nm. This is because under the Q-switched process, thefiber is very strongly pumped and, under these conditions, the use of a980 nm pump wavelength would result in severe losses due to a two photonupconversion process (excited state absorption—ESA).

In FIG. 1E is shown a short fiber amplifier structure similar to that ofthe short fiber lasers shown in FIGS. 1A-1D. Unlike the fiber laserembodiments, the signal wavelength is not generated within the gainmedium, but is coupled together with the pump wavelength using anoptical coupler such as WDM 17. Pump resonator, established by gratings14, keeps the pump energy oscillating within the gain medium, where itcan be absorbed by the doped fiber. This resonant pumping maintains apopulation inversion in the amplifier gain medium. Thus, the signalenergy is amplified through stimulated emission from the pumped fiber.

FIG. 1F depicts an embodiment of the present invention in which resonantpumping is used to provide population inversion in an amplifiedspontaneous emission (ASE) source. Like the above embodiments, the ASEsource uses doped optical fiber 10 and a pump resonator consisting ofgratings 14. Optical pumping source provides the pump energy whichresonates between the gratings 14, being absorbed by the optical fiber.However, rather than providing a single wavelength signal resonancecavity, as in the fiber laser embodiments, the pumped fiber 10 is simplyallowed to generate a broadband output. A broadband grating 21 islocated between the pump source 15 and the fiber gratings 14 to ensurethat the ASE output (indicated in the figure by the symbol λ_(ASE) isdirected in the direction away from the pump source 15.

In each of the embodiments of FIGS. 1A, 1B 1C and 1D, the wavelengthsused are selected based on the desired signal output and the dopingmaterial used for the fiber 10. For example, a preferred embodiment maybe designed for an optical signal at a wavelength of λ_(s)=1550 nm,which is near the transmission peak of conventional silica-based opticalfiber. In such a case, an erbium-doped optical fiber may be used, andmay be pumped with optical energy at a wavelength of, e.g., Ap 980 nm.Similarly, an erbium/ytterbium doped fiber could be used, and could bepumped with optical energy at a wavelength of, e.g. λ_(p) _(—) 1060 nm.

Referring to FIGS. 2A-2C, a tunable embodiment of the invention is shownin which two short fiber lasers are arranged in series. In FIG. 2A, twolasers each having the construction of the laser of FIG. 1A are formedby different gratings etched into doped optical fiber 10. Fiber laser 11a is constructed in the fiber from signal grating 12 a and pump grating14 a, while fiber laser 11 b is constructed from signal grating 12 b andpump grating 14 b. The signal gratings 12 a, 12 b are bothwavelength-selective, each being reflective at a different wavelengthwithin the gain profile of the doped optical fiber. For example, iffiber 10 is doped with erbium/ytterbium, the signal grating 12 a may beconstructed to be highly reflective at a wavelength of λ_(s1)=1552 nm,while grating 12 b is constructed to be highly reflective at awavelength of λ_(s2)=1550 nm. Likewise, the pump gratings 14 a, 14 b arealso wavelength selective and each reflective at a different wavelengthwithin the acceptable range of pump wavelengths for the fiber. For anerbium/ytterbium doped fiber 10, pump grating 14 a may be made highlyreflective at a wavelength of λ_(p1)=1062 nm, while pump grating 14 b ishighly reflective at a wavelength of λ_(p2)=1060 nm.

The wavelength discrimination of each signal grating is high enough thatoptical signal energy at the wavelength of one grating is notsignificantly affected by the other grating. For example, optical energyat λ_(s1), while being reflected by grating 12 a, bypasses grating 12 bwith only a small amount of loss due to interaction between the lightand the grating 12 b. The same is true for the wavelength discriminationof the pump gratings. That is, optical energy at wavelength λ_(p1)suffers only a relatively small amount of loss due to interaction withthe pump gratings 14b. This high discrimination of the gratings 12 a, 12b, 14 a, 14 b allows the overall laser device to have a selectableoutput wavelength.

The input to the laser embodiment of FIG. 2A is a tunable pump energysource 19, which may be tuned to a wavelength of either λ_(p1), orλ_(p2). When the input pump source is tuned to λ_(p1), the pump energyis reflected within the gain medium of short fiber laser 11 a by pumpgrating 14 a. This reflection keeps most of the pump energy resonatingwithin the cavity of laser 11 a, and allows it to be absorbed by thefiber, which then generates optical energy through stimulated emission.The optical energy output within the cavity includes optical energy atthe signal wavelength λ_(s1), which is within the range of outputwavelengths for doped optical fiber 10. Since signal grating 12 a ishighly reflective at λ_(s1), a resonance condition at this wavelength isestablished within the cavity of short fiber laser 11 a. The grating 12a is selected to allow a certain amount of transmission in the outputdirection of the laser 11 a (i.e. toward the right at the orientationshown in FIG. 2A). Thus, laser energy at wavelength λ_(s1), is outputfrom the short fiber laser 11 a toward short fiber laser 11 b. However,because of the high degree of discrimination of the gratings of bothlasers, the desired λ_(s1) output passes through the laser 11 b withonly a small degree of loss due to reflection from grating 12 b.Similarly, any λ_(p1) pump energy leaking out of the first short fiberlaser 11 a does not significantly interact with gratings 14 b of laser11 b, and pumping of the second laser cavity is therefore minimized.

When an output at wavelength λ_(s2) is desired, the wavelength of theinput optical energy is set at λ_(p2.) Because of the highdiscrimination of the gratings, very little loss of the λ_(p2) pumpenergy occurs as it passes through fiber laser 11 a. However, the highreflectivity of gratings 14 b result in the λ_(p2) pump energyresonating within the gain medium of fiber laser 11 b. Absorption of thepump energy results in stimulated emission within the cavity of laser 11b, including emission of optical energy at wavelength λ_(s2). Thisenergy is reflected within the cavity of laser 11 b due to the highreflectivity of signal grating 12 b, and a resonance condition isestablished. The signal grating 12 a is selected to allow partialtransmission of the λ_(s2) optical energy out of the laser 11 b cavity,providing the necessary output coupling of the laser. Thus, the laseroutput may be selected to be either λ_(s1) or λ_(s2) by tuning thewavelength of the input pump energy.

The embodiments of FIGS. 2B and 2C function in essentially the same wayas the embodiment of FIG. 2A, except for the specific construction ofeach of the fiber lasers 11 a, 11 b. In FIG. 2B, short fiber lasers 11a, 11 b each have the same construction as the short fiber laser of FIG.1B, that is, each has a single signal grating (12 a and 12 b,respectively) which extends the length of the gain medium, and two pumpgratings (14 a and 14 b, respectively) at the ends of the gain medium.For each laser 11 a, 11 b, the signal grating is partially reflectivetoward the output side of the laser (i.e. to the right in theorientation of FIG. 2B) to provide the output coupling of each laser. Asin the embodiment of FIG. 2A, the wavelength discrimination of thegratings is high enough that there is very little interference with onesignal wavelength by the pump grating constructed to reflect the othersignal wavelength. Similarly, there is very little interference with onepump wavelength by the pump gratings constructed to reflect the otherpump wavelength. By selecting between input pump wavelengths λ_(p1),λ_(p2), one may select between output signal wavelengths λ_(s1), λ_(s2).

The embodiment of FIG. 2C also functions in the same manner as theembodiment of FIG. 2A, except that the short fiber lasers 11 a, 11 bhave a construction like that of the laser shown in FIG. 1C (oralternatively use the Q-switched embodiment of FIG. 1D). That is, ineach laser 11 a, 11 b, signal gratings (12 a and 12 b, respectively) areto either side of the gain medium, as are two pump gratings (14 a and 14b, respectively). When input wavelength λ_(p1) is selected, the gratings14a reflect the pump energy back and forth across the gain medium ofshort fiber laser 11 a, allowing the absorption of pump energy andstimulated emission of signal energy. The signal grating 12 a to theoutput side of the laser (i.e. to the right in the orientation of FIG.2C) has a degree of partial reflectivity which allows some of the signalenergy to be coupled out of the laser. When the input wavelengthselected is λ_(p2), the pump energy substantially bypasses laser 11 a,and is reflected between pump gratings 14 b of laser 11 b, resulting inits absorption and the generation of optical energy at wavelength 42.The signal grating 1 2b to the output side of laser 11 b (i.e. to theright in the orientation of FIG. 2C) is partially transmissive to allowoutput coupling of the λ_(s2) laser energy. Thus, as in the embodimentsof FIGS. 2A and 2B, selection between input pump wavelengths λ_(p1),λ_(p2) with source 19 allows one to select between output signalwavelengths λ_(s1), λ_(s2).

For each of the embodiments of FIGS. 2A-2C, it is preferable that thesignal grating(s) of the laser 11 a be constructed to reflect longerwavelengths than the signal grating(s) of laser 11 b. This is due to thefact that the interference encountered by shorter wavelengths passingthrough gratings set to reflect higher wavelengths is less than theinterference encountered by longer wavelengths passing through gratingsarranged to reflect shorter wavelengths. Thus, the series arrangement ofshort fiber lasers in this manner should always have lasers withgratings constructed to reflect longer wavelengths preceding those withgratings constructed to reflect shorter wavelengths.

It should also be readily apparent to one skilled in the art that thetunable laser configurations of FIGS. 2A-2C should not be consideredlimited to only two lasers. While only two lasers are shown in each ofthese figures, additional short fiber lasers with different gratings canbe placed in series with lasers 11 a, 11 b to allow additional tunableoutput wavelengths. Furthermore, the embodiments of FIGS. 2A-2Cincorporate the resonant pumped laser structures shown in FIGS. 1A-1Dbut, in each embodiment, use the same type of structure for both of theseries-linked lasers. Those skilled in the art will recognize that thedifferent laser structures shown in FIGS. 1A-1D can be mixed and matchedto arrive at a desired configuration. For example, a series combinationof the structure of FIG. 1A and the structure of FIG. 1C could be used.

Depicted in FIGS. 3A and 3B are variations of the wavelength selectiveembodiments of FIGS. 2B and 2C in which the output signals for each ofthe lasers 11 a, 11 b are extracted directly from the lasers themselvesusing wavelength division multiplexers. The arrangement shown in FIG. 3Ais the same as that of FIG. 2B, except that wavelength divisionmultiplexer (WDM) 16a is located between signal grating 12 a and theoutput side pump grating 14 a of the laser 11 a. The WDM 16 a divertsoptical energy at a wavelength of λ_(s1) to a desired output location.While WDM 16 a is only schematically shown in the figure, the generaluse of WDMs is well-known in the art. By using WDM 16 a as the outputpath for the laser energy of short fiber laser 11 a, the λ_(s1), laseroutput does not have to pass through short fiber laser 11 b. Thus, anylosses due to interaction with the gratings of laser 11 b are avoided.Obviously, the pump energy λ_(p1), which pumps laser 11 a, is notdiverted by the WDM 16 a as it reflects back and forth through the gainmedium of laser 11 a. Likewise, pump energy at the wavelength 42 passesthrough the WDM to pump the fiber laser 11 b.

In the fiber laser 11 b of FIG. 3A, WDM 16 b is used to divert the laseroutput at wavelength λ_(s2) to a desired output location. As in laser 11a, the pump energy is not diverted, and remains within the laser cavity,reflecting between the gratings 14 b. Those skilled in the art willrecognize that the WDM 16 b could be omitted if the system had only thetwo lasers 11 a, 11 b, and there would be little difference in theλ_(s2) signal loss. However, WDM 16 b is included in FIG. 11b since theembodiment may also include additional short fiber lasers havingdifferent output wavelengths. If these additional fiber lasers arelocated to the output side of laser 11 b (i.e. to the right of laser 11b relative to the orientation of FIG. 3B), WDM 16 b prevents the λ_(s2)output signal from having to pass through the signal gratings of thesubsequent lasers, where some losses might occur.

The configuration of FIG. 3B is identical to that of FIG. 2C, exceptthat WDMs 16 a, 16 b are used in the same manner as shown in FIG. 3A.That is, the WDMs 16 a, 16 b are used in the FIG. 3B embodiment todivert the laser signal energy individually out of each laser to avoidany losses which might occur as it passes through downstream lasercomponents. Again, the WDM 16 b is included to allow for the diversionof signal energy at a wavelength of λ_(s2) should additional short fiberlasers be used. In each of the embodiments of FIGS. 3A and 3B, thearrangement provided allows for a selectable laser output which ischanged by changing the input pump wavelength, as is described abovewith regard to FIGS. 2A-2C. A further embodiment can also be used whichfunctions in the same manner as the embodiments of FIGS. 3A and 3B, butwhich uses a laser structure like that of FIG. 1B in one stage, whileusing a laser structure like that of FIG. 1C in the other stage.

Because the resonant pumping laser systems described herein rely on ahigh degree of wavelength discrimination for performance, accuratecontrol of the light input to the lasers can be very important. This isparticularly true when using light sources that can drift in wavelength.For example, sources such as semiconductor lasers are known to sufferdrift of their output wavelength with changes in time and/ortemperature.

Shown in FIG. 4A is a fiber laser system which uses electronic feedbackto maintain its wavelength stability of the pump source. The source ofpumping energy is a single-wavelength semiconductor pump laserconsisting of semiconductor diode 20 and doped optical fiber 22 (forease of description, optical fibers in FIGS. 4A and 4B are each shownschematically as a line). Such pump lasers are well-known in the art,and provide a single wavelength optical output. However, changes in timeand temperature may cause the pump laser output to drift slightly inwavelength. Since this laser source is to be used as a pump source for aresonant-pumped short fiber laser having reflectors at the peak pumpwavelength, it is important to prevent such drift so that the output ofthe pump laser remains stable at the desired pump wavelength. To dothis, the embodiment of FIG. 4A uses electronic feedback to control thecurrent driving the diode, as described in more detail below. Sincechanges in the diode driver current cause changes in the outputwavelength of the diode, this feedback may be used to combat wavelengthdrift in the pump laser.

The semiconductor diode 20 is current driven, and has a DC power inputprovided by DC voltage source. The power input to the diode is alsomodulated by AC voltage source 36, the amplitude of which is a fraction(e.g. several percent) of the DC voltage from voltage source 36.Inductor 40 and capacitor 42 help prevent any interaction between thetwo sources 36, 38. Thus, the electrical power to the diode 20 (andtherefore the optical power output) is modulated about a particularmagnitude at the frequency of the AC voltage source 36. This modulationresults in an FM signal with two small side bands in addition to thecarrier of the unmodulated pump laser.

The output of doped fiber 22 is coupled into optical isolator 22. Theoptical isolator 22 is a unidirectional optical element that is known inthe art and commercially available. The isolator 22 prevents any opticalenergy from being fed back to the diode and disrupting its operation.From the isolator 22, the pump energy passes to optical coupler 26.Optical coupler 26 is some type of beamsplitter, such as a wavelengthdivision multiplexer. It passes most of the optical energy through toshort fiber laser 11, which consists of signal gratings 12 and pumpgratings 14. The fiber laser 11 shown in FIG. 4A is the sameconstruction as that of FIG. 1C. However, the electronic feedbackarrangement shown may be as easily used with the resonant pumped laserarrangements shown in FIGS. 1A and 1B.

The optical energy provided by the semiconductor pump laser is reflectedbetween pump gratings 14 as described in previous embodiments. Thisresonance keeps the pump energy in the gain medium where it is absorbedby the fiber, which then outputs optical energy at the desired signalwavelength by stimulated emission. The combination of gratings 12, 14 tothe input side of the fiber laser 11 also reflects a small amount (e.g.5%) of the pump energy reaching it. When the pump energy input to thefiber laser 11 is at the correct wavelength, the peak resonant conditionfor the gratings is achieved. However, if the input energy is “offresonance” (i.e. the wavelength is different from that for which thegratings 14 are constructed), the optical energy reflected back towardoptical coupler 26 will have an increased intensity. In addition to theincrease in reflectivity, a non-resonant condition will result in adifferent phase shift for the two side bands of the FM signal. Thisphase shift, in turn, results in an FM-to-AM conversion. The magnitudeof the AM signal is a measure of how far the pump laser isoff-resonance. This may then be used to provide the desired feedbackcontrol of the diode 20.

After reflection from the input pump grating 14 of laser 11, thisreflected pump energy returns to the directional coupler, and a portionof it is directed to filter/attenuator 28. Filter/attenuator 28 is anoptional component which may be used to block wavelengths outside of thedesired range of the pump wavelength. This minimizes the relative noisein the reflected pump energy. Filter/attenuator 28 may also simplyreduce the magnitude of the feedback signal to a desirable level. Afterexiting the filter/attenuator 28, the reflected pump energy is input tophotodetector 30, which converts the optical energy into an electricalfeedback signal.

The electrical feedback signal has the characteristics of the reflectedoptical signal, including any intensity change and phase shift. Thissignal is amplified by linear amplifier 32, and is input to mixer 34.Mixer 34 is an electronic frequency mixer, as is know in the art, andcombines the amplified feedback signal with the AC signal from ACvoltage source 36. Because the frequencies of the signals are identicalor nearly identical, the output from mixer 34 is a DC voltage. Thisvoltage is amplified using amplifier 44. The magnitude of the DC voltageis affected by the relative phase of the feedback signal and the ACinput signal. This phase difference is, in turn, affected by thedifference between the resonant wavelength of the pump gratings 14(i.e., the wavelength at which a peak resonance condition is achieved bythe gratings 14) and the wavelength of the optical energy from thediode. If the output wavelength of the diode drifts shorter, therelative phase shift will cause the DC signal output from mixer 34 todecrease in magnitude. This, in turn, causes the wavelength output fromthe diode to increase, moving it back toward the peak resonancecondition of the gratings 14. Similarly, if the output wavelength of thediode becomes longer, the DC signal output from mixer 34 increases inmagnitude. This causes the wavelength of the diode optical output todecrease, again moving it toward the peak resonance condition of thegratings 14.

FIG. 4B shows another feedback arrangement for use with the resonantpumped short fiber lasers disclosed herein. However, rather than usingelectronic feedback, as in the embodiment of FIG. 4A, the FIG. 4Bembodiment uses optical feedback. The optical source of pumping energyis again a fiber laser consisting of semiconductor laser diode 20 anddoped fiber coil 22. The pumping energy is input to optical circulator46. Optical circulator 46 is a device which is known in the art andwhich is commercially available. The optical circulator is aunidirectional optical element which may have multiple inputs andoutputs. The pump energy from fiber coil 22 is input to the circulator,but is output only at the input to short fiber laser 11. Since thecirculator 46 is unidirectional, no optical transmission can pass backthrough the circulator from laser 11 to fiber coil 22. Similarly, anyoptical energy which is output by filter/attenuator 50 (discussedfurther below) is directed only to the coil 22, with no optical energybeing allowed to go the opposite direction.

The optical pump energy exiting circulator 46 is introduced to the fiberlaser 11, and is reflected between the pump gratings 14, allowing it tobe absorbed by the doped fiber of the laser. Again, the fiber laser 11is the same construction as the laser shown in FIG. 1 C but, for thepurposes of this feedback embodiment, could just as well be either ofthe embodiments shown in FIGS. 1A and 1B. For pump energy which is “onresonance” (i.e. which is at the peak reflective wavelength of thegratings 14) there is virtually no loss of optical pump energy enteringthe fiber laser. However, the further off resonance the wavelength ofthe input pump energy drifts, the more of it gets reflected back towardthe circulator. This, in turn, reduces the amount of pump energyentering the fiber laser cavity.

The pump grating 14 toward the output side of the laser 11 (i.e. towardthe right in the orientation of FIG. 4B) allows a small fraction of“on-resonance” pump energy to be transmitted past it. This pump energy,along with the signal energy coupled past the partially transmissiveoutput signal grating 12, is directed to optical coupler 48. Opticalcoupler 48 is preferably a wavelength division multiplexer, and directsoptical energy in the wavelength range of the pump energy towardfilter/attenuator 50, while directing energy in the wavelength range ofthe laser 11 signal output, λ_(s), toward an output port of the laser.The pump energy reaching filter/attenuator 50 may be bandpass filteredto eliminate excess noise in the returning pump signal, or may beattenuated to prevent a feedback signal of too high a magnitude. Thefilter/attenuator 50 is a device known in the art, and is optionaldepending on the specific design specifications of the system.

The pump energy output from the wavelength division multiplexer 48constitutes an optical feedback signal, and is used to control theoutput of the fiber laser source consisting of diode 20 and dopedoptical fiber 22. After passing through the filter/attenuator 50, thefeedback signal passes through circulator 46 toward the source laser.The interaction of the feedback signal with the laser causes it toresonate at the wavelength of the feedback signal. That is, the laser“locks on” to the wavelength of the feedback signal. Since thewavelength of the feedback signal is the “on-resonance” wavelength ofthe pump gratings 14 of short fiber laser 11, this forces the outputfrom source laser 20, 22 to match the peak resonance wavelength of thepump gratings 14, and thus prevents the wavelength drift of the pumpsource.

Shown in FIG. 5A is an arrangement for pumping a plurality of shortfiber lasers simultaneously. An optical pumping source, such assemiconductor laser diode 60, provides optical energy at a desiredwavelength. For a fiber medium doped with erbium, laser source 60, whichmay be a semiconductor laser diode, would typically have a wavelength ofapproximately 980 nm. The output of diode 60 is input to an opticalsplitter 62. Splitter 62 is an optical element of known design, such asa fused fiber directional coupler, that divides the input signal fromdiode 60 into a plurality of outputs, each of the outputs receiving thesame optical power. That is, the splitter provides a equal division ofthe input signal optical power to each of its multiple outputs.

Each outputs of splitter 62 is directed to one of a number of shortfiber lasers. Each fiber laser includes a highly reflective grating 64a-64 h (generally referred to as 64) and a partially reflective grating66 a-66 h (generally referred to as 66) that serves as the laser outputcoupler. In the preferred embodiment, the gratings 64, 66 of each of thefiber lasers are constructed to resonate at a different wavelength thanthat of the other lasers. In this way, the arrangement provides aplurality of different output signals (eight in the embodiment shown),each of which has a different wavelength (the different outputwavelengths are indicated in FIG. 5A as λ₁-λ₈). Furthermore, each of thelaser signals has the same output power as the others, thereby providinga user with a good set of optical signals for multiplexed communicationsapplications.

In FIG. 5B is depicted an embodiment in which the outputs from aplurality of short fiber lasers are all coupled into a single fiber. Asin FIG. 5A, the embodiment of FIG. 5B uses a plurality of short fiberlasers, each of which resonates at a different wavelength (in thefigure, the different wavelengths are indicated as λ₁-λ₈). However, inthe embodiment of FIG. 5B, the partially reflective gratings 66 a-66 h,which function as the laser output couplers, are to the side of thelaser facing optical splitter 62, while the highly reflective gratings64 a-64 h are to the side of the laser away from splitter 62. The lasersare pumped by an optical source, such as semiconductor laser diode 60,which is coupled into the system output fiber via WDM 68. The opticalpower of the pumping signal is split between the different fiber lasersby splitter 62, such that the pumping power to each of the lasers isapproximately equal. As the different fiber lasers resonate at theirdifferent respective wavelengths, the output coupling gratings 66 of thelasers direct the laser outputs back through the splitter 62 and intothe output fiber. The wavelengths output from the lasers aresignificantly different than the pumping wavelength and are directedtogether by WDM 68 to output port 70.

The feedback arrangements of FIGS. 4A and 4B may also be applied to themultiple short fiber laser embodiments of FIGS. 5A and 5B. Multiplelaser embodiments using electronic feedback are shown in FIGS. 6A and6B, FIG. 6A being essentially a combination of the embodiments of FIG.4A and FIG. 5A, while FIG. 6A is essentially a combination of theembodiments of FIG. 4A and FIG. 5B. In each of the FIG. 6A and FIG. 6Bembodiments, it is only necessary to return optical feedback energy fromone of the short fiber laser branches, since the resonator modes of thedifferent lasers are close in wavelength. Preferably, the laser fromwhich optical energy is fed back is one in the middle of the wavelengthrange of the different lasers.

In each of FIGS. 6A and 6B, reference numerals are used which matchthose of earlier figure. In each of these figures, the lasers are alsoresonant pumped, having pump reflector gratings 63 a-63 h and 65-65 h inaddition to signal reflector gratings 64 a-64 h and 66 a-66 h. In FIG.6A, the output wavelength of pumping source 60 is controlled usingelectronic feedback in the same manner as described above with regard toFIG. 4A. The optical reflection signal is taken from the fiber laserhaving gratings 63 a, 64 a, 65 a, 66 a. This arrangement of lasers isthen adequately pumped to produce the desired multiple outputs λ₁-λ₈. InFIG. 6B, the same type of electronic feedback control is used, and theoptical feedback signal is again taken from the laser resonator definedby gratings 63 a, 64 a, 65 a, 66 a. This allows stable pumping in thedesired range, and provides the desired combined output of wavelengthsλ₁-λ₈. The embodiments of FIGS. 7A and 7B are similar to those of FIGS.6A and 6B, except that they use an optical feedback arrangement likethat shown in FIG. 4B, rather than the electronic feedback version. Thereference numerals in FIGS. 7A and 7B match those of earlier figures,where appropriate. As in the embodiments of FIGS. 6A and 6B, theembodiments of FIGS. 7A and 7B use resonant pumping with pump gratings63 a-63 h and 65 a-65 h. In FIG. 7A, some of the output from the laserdefined by gratings 63 a, 64 a, 65 a, 66 a is returned to the fiberlaser source via WDM 48, aftenuator 50 and optical circulator 46. Thisstabilizes the output wavelength of the optical source, so that all ofthe lasers are adequately pumped, and produce the desired multipleoutputs having wavelengths λ₁-λ₈, respectively. In FIG. 7B, part of theoutput from the laser defined by gratings 63 a, 64 a, 65 a, 66 a isreturned to the source via WDM 48, attenuator 50 and optical circulator46 and stabilizes the output wavelength of the source. This allows thepumping wavelength to be sufficient for all of the lasers, the outputsof which are combined to create the output at port 70 consisting ofwavelengths λ₁-λ₈.

While the invention has been shown and described with regard to apreferred embodiment thereof, it will be recognized by those skilled inthe art that various changes in form and detail may be made hereinwithout departing from the spirit and scope of the invention as definedby the appended claims. For example, while the concepts above have beendiscussed in the context of optical fiber waveguides, they may be easilyextended to waveguide resonators in other media including, but notlimited to, planar silica-based (SiO₂) waveguides that are formed onsilicon substrates, or lithium niobate (LiNbO₃) passive or activewaveguides formed on LiNbO₃ substrates.

What is claimed is:
 1. A wavelength-stabilized optical source forgenerating a wavelength-stable optical signal, the source comprising: anoptical generator generating optical source energy having a timevaryingintensity and directing it in a first direction along an optical path; aresonant pumping cavity toward which the optical source energy isdirected and which resonates optical pumping energy through an opticalgain medium while reflecting a portion of the source energy along theoptical path in a direction opposite the first direction such that thereflected optical energy interacts with the source energy, thereflection being such that a phase shift between the time-varyingintensity of the optical source energy and a time-varying intensity ofthe reflected optical energy is minimized when the source energy has aparticular primary wavelength; a detector that detects optical energythat has been reflected by the resonant cavity and has interacted withthe source energy, and converts it to a feedback signal indicative ofsaid phase shift; and a feedback apparatus that provides an input signalto the optical generator in response to the feedback signal, the inputsignal causing the output wavelength of the optical generator to beadjusted toward the particular wavelength.
 2. An optical sourceaccording to claim 1 further comprising a DC source and an AC signalsource that both contribute to the input signal.
 3. An optical sourceaccording to claim 2 wherein the feedback apparatus comprises a mixerthat combines the feedback signal and a signal from the AC signal sourceto generate a DC offset signal that contributes to the input signal. 4.An optical source according to claim 3 wherein, when said phase shifthas a non-zero value in a first phase direction, the DC offset signalhas a negative value, thereby decreasing the total DC signal input tothe optical generator, and when said phase shift has a non-zero value ina second phase direction, the DC offset signal has a positive value,thereby increasing the total DC signal input to the optical generator.5. An optical source according to claim 2 further comprising anisolation apparatus that isolates the DC source from the AC signalsource.
 6. An optical source according to claim 1 further comprising anoptical coupler that directs the reflected optical energy to thedetector.
 7. An optical source according to claim 6 wherein the opticalcoupler comprises a wavelength division multiplexer.
 8. An opticalsource according to claim 1 further comprising an optical filter forblocking, in the reflected optical energy, wavelengths outside of acertain range surrounding the particular wavelength.
 9. An opticalsource according to claim 1 further comprising an optical isolatorthrough which the optical signal from the optical generator passes priorto reaching the resonant cavity.
 10. An optical source according toclaim 1 wherein the optical generator comprises an optical fiber gainmedium.
 11. An optical gain apparatus comprising: an optical pump sourcegenerating optical pump energy having a time-varying intensity anddirecting it in a first direction along an optical path; an optical gainmedium capable of being pumped by optical energy at a pump wavelength,the gain medium receiving optical pump energy directed in the firstdirection along the optical path; a resonant pumping cavity thatresonates optical pumping energy through the optical gain medium whilereflecting a portion of the pump energy that is directed toward the gainmedium, the reflected energy following a direction opposite the firstdirection such that the reflected optical energy interacts with the pumpenergy, the reflection being such that a phase shift between thetime-varying intensity of the optical pump energy and a time-varyingintensity of the reflected optical energy is minimized when the pumpenergy has a primary wavelength equal to the pump wavelength; a detectorthat detects optical energy that has been reflected by the resonantpumping cavity and has interacted with the pump energy, and converts itto a feedback signal indicative of said phase shift; and a feedbackapparatus that provides an input signal to the optical pump source inresponse to the feedback signal, the input signal causing the outputwavelength of the optical source to be adjusted toward the pumpwavelength.
 12. An optical gain apparatus according to claim 11 furthercomprising a DC source and an AC signal source that both contribute tothe input signal.
 13. An optical gain apparatus according to claim 12wherein the feedback apparatus comprises a mixer that combines thefeedback signal and a signal from the AC signal source to generate a DCoffset signal that contributes to the input signal.
 14. An optical gainapparatus according to claim 14 wherein, when said phase shift has anon-zero value in a first phase direction, the DC offset signal has anegative value, and decreases a total DC signal input to the pumpsource, and when said phase shift has a non-zero value in a second phasedirection, the DC offset signal has a positive value, and increases thetotal DC signal input to the pump source.
 15. An optical gain apparatusaccording to claim 11 wherein the resonant cavity contains opticalenergy at the pump wavelength.
 16. An optical gain apparatus accordingto claim 11 wherein the optical pump source comprises an optical fibergain medium.
 17. An optical source for providing a wavelength-stabilizedoptical output to a resonant pumping cavity that resonates opticalpumping energy through an optical gain medium and from which a smallamount of optical energy is reflected back toward the source, thereflection being such that a phase shift between a time-varyingintensity of the optical energy directed toward the resonsant pumpingcavity and a time-varying intensity of the reflected optical energy isminimized when a primary wavelength of the optical energy directedtoward the resonant pumping cavity is centered about a peak reflectivitywavelength of the resonant pumping cavity, the optical sourcecomprising: an optical signal generator generating optical source energyhaving a time-varying intensity and directing it toward the resonantpumping cavity; a detector that detects optical energy reflected fromthe cavity and converts it to a feedback signal indicative of said phaseshift; and a feedback apparatus that provides an input signal to theoptical signal generator in response to the feedback signal, the inputsignal causing the output wavelength of the optical signal generator tobe adjusted toward the particular wavelength.
 18. An optical sourceaccording to claim 17 wherein the optical signal generator comprises adoped optical fiber and a semiconductor diode.
 19. An optical sourceaccording to claim 17 wherein a time-varying input signal is input tothe optical signal generator that comprises contributions from a DCsource and an AC signal source.
 20. An optical source according to claim19 wherein the feedback apparatus comprises an mixer that combines thefeedback signal with an AC current from the AC signal source to generatea DC offset signal input to the optical signal generator.
 21. An opticalsource according to claim 20 wherein, when said phase shift has anon-zero value in a first phase direction, the DC offset signal has anegative value, thereby decreasing the total DC signal input to theoptical signal generator, and when said phase shift has a non-zero valuein a second phase direction, the DC offset signal has a positive value,thereby increasing the total DC signal input to the optical signalgenerator.
 22. An optical source according to claim 19 furthercomprising an apparatus that isolates the DC source from the AC signalsource.
 23. An optical source according to claim 17 further comprisingan optical filter for blocking, in the reflected optical energy,wavelengths outside of the particular wavelength range from reaching thedetector.
 24. An optical source according to claim 17 further comprisingan optical coupler that directs the reflected optical energy to thedetector.
 25. An optical source according to claim 17 further comprisingan optical isolator through which the optical signal from the opticalsignal generator passes prior to reaching the device.
 26. An opticalsource according to claim 17 wherein the optical signal generatorcomprises an optical fiber gain medium.
 27. A method for generating awavelength-stable optical signal, the method comprising: generating,with an optical generator, optical source energy having a timevaryingintensity and directing it in a first direction along an optical path;reflecting a portion of the source energy from a resonant pumping cavitythat pumps a gain medium. the reflection being in a direction oppositethe first direction such that the reflected optical energy interactswith the source energy, the reflection being such that a phase shiftbetween the time-varying intensity of the optical source energy and atime-varying intensity of the reflected optical energy is minimized whenthe source energy has a particular primary wavelength; detecting opticalenergy that has been reflected by the resonant pumping cavity and hasinteracted with the source energy, and converting it to a feedbacksignal indicative of said phase shift; and directing the feedback signalto a feedback apparatus that provides an input signal to the opticalgenerator in response to the feedback signal, the input signal causingthe output wavelength of the optical generator to be adjusted toward theparticular wavelength.
 28. A method according to claim 27 furthercomprising generating a DC signal and an AC signal that contribute tothe input signal.
 29. A method according to claim 28 further comprisingcombining the feedback signal and a predetermined AC signal with a mixerto generate a DC offset signal that contributes to the input signal. 30.A method according to claim 29 wherein, when said phase shift has anon-zero value in a first phase direction, the DC offset signal has anegative value, thereby decreasing the total DC signal input to theoptical generator, and when said phase shift has a non-zero value in asecond phase direction, the DC offset signal has a positive value,thereby increasing the total DC signal input to the optical generator.31. A method according to claim 27 wherein generating optical sourceenergy comprises generating optical source energy with an opticalgenerator that uses a optical fiber gain medium.
 32. A method ofproviding an amplified optical signal output, the method comprising:generating, with an optical pump source, optical pump energy having atime-varying intensity and directing it in a first direction along anoptical path toward a resonant pumping cavity that resonates opticalenergy at a pump wavelength through an optical gain medium; reflecting aportion of the pump energy that is directed toward the resonant pumpingcavity, the reflected energy following a direction opposite the firstdirection such that the reflected optical energy interacts with the pumpenergy, the reflection being such that a phase shift between thetime-varying intensity of the optical pump energy and a time-varyingintensity of the reflected optical energy is minimized when the pumpenergy has a primary wavelength equal to the pump wavelength; detectingoptical energy that has been reflected and has interacted with the pumpenergy, and converting it to art feedback signal indicative of saidphase shift; and providing an input signal to the optical pump source inresponse to the feedback signal so as to cause the output wavelength ofthe optical pump source to be adjusted toward the pump wavelength.
 33. Amethod according to claim 32 further comprising contributing to theinput signal with a DC signal and an AC signal.
 34. A method accordingto claim 33 further comprising combining the feedback signal with the ACsignal to generate a DC offset signal that contributes to the inputsignal.
 35. A method according to claim 34 wherein, when said phaseshift has a non-zero value in a first phase direction, the DC offsetsignal has a negative value, thereby decreasing a total DC signal inputto the pump source, and when said phase shift has a non-zero value in asecond phase direction, the DC offset signal has a positive value,thereby increasing the total DC signal input to the pump source.
 36. Amethod according to claim 32 wherein generating optical pump energycomprises generating optical pump energy with an optical pump sourcethat comprises an optical fiber gain medium.
 37. A method of providingwavelength-stabilized optical energy to a resonant pumping cavity thatpumps an optical gain medium and from which a small amount of theoptical energy is reflected, the reflection being such that a phaseshift between a time-varying intensity of the optical energy directedtoward the resonant pumping cavity and a time-varying intensity of thereflected optical energy is minimized when a primary wavelength of theoptical energy directed toward the resonant pumping cavity is centeredabout a particular wavelength, the method comprising: generating, withan optical signal generator, optical source energy having a time-varyingintensity and directing it toward the resonant pumping cavity; detectingoptical energy reflected from the resonant pumping cavity and convertingit to a feedback signal indicative of said phase shift; and providing aninput signal to the optical signal generator in response to the feedbacksignal, the input signal causing an output wavelength of the opticalsignal generator to be adjusted toward the particular wavelength.
 38. Amethod according to claim 37 wherein generating optical source energycomprises generating optical source energy with an optical signalgenerator that comprises a doped optical fiber and a semiconductordiode.
 39. A method according to claim 38 wherein the input signalcomprises contributions from a DC source and an AC signal source.
 40. Amethod according to claim 39 further comprising combining the feedbacksignal with the AC signal to generate a DC offset signal thatcontributes to the input signal.
 41. A method according to claim 40wherein, when said phase shift has a non-zero value in a first phasedirection, the DC offset signal has a negative value, thereby decreasingthe total DC signal input to the optical signal generator, and when saidphase shift has a non-zero value in a second phase direction, the DCoffset signal has a positive value, thereby increasing the total DCsignal input to the optical signal generator.
 42. A method according toclaim 37 wherein generating optical source energy comprises generatingoptical source energy with an optical signal generator that comprises anoptical fiber gain medium.